1. Field of the Invention
The present invention relates in general to a switching element. More particularly, the present invention relates to a two-terminal nonlinear element as an exemplary switching element, a liquid crystal display device including such an element and a method for fabricating the same.
2. Description of the Related Art
In recent years, liquid crystal display devices have been used for display applications in personal computers, word processors, office automation terminal units, television receivers and the like, because a liquid crystal display device is a small-sized light-weight device consuming little power. As liquid crystal display devices become more and more popular, it becomes more and more necessary for them to display an image of higher quality and to increase the display capacity thereof.
A conventional liquid crystal display device has been driven in a simple matrix manner by a voltage averaging method in Twisted Nematic (TN) mode or Super Twisted Nematic (STN) mode. However, since such a method requires a larger number of scanning lines, the contrast ratio thereof may become unsatisfactory. Thus, such a method is not suitable for large-capacity display.
In order to deal with such a problem, active driving has been developed by providing a switching element for each of the pixels on a display screen. Switching elements used for such a purpose include thin-film transistors and two-terminal nonlinear elements. Comparing these two types of elements, the two-terminal nonlinear elements are more advantageous in respects of the simplified configuration and the reduced fabrication costs thereof. Thus, a liquid crystal display device using such two-terminal nonlinear elements is expected to be a mainstream product in the near future. Reflecting these tendencies, a nonlinear element having a metal-insulator-metal structure (hereinafter, such an element will be simply referred to as an "MIM element") has already been in practical use.
The MIM element exhibits current-voltage characteristics following a so-called Poole-Frenkel rule. More specifically, when the voltage of an input signal is low, the resistance of such an element becomes high. On the other hand, when the voltage of the input signal is sufficiently high for driving liquid crystal molecules, the resistance of such an element becomes low.
In a liquid crystal display device including the MIM elements, such nonlinear current-voltage characteristics are applied to switching the ON/OFF states of the elements.
FIG. 23 is a plan view of a conventional liquid crystal display device 1 including MIM elements. In FIG. 23, a substrate 2 on which MIM elements are formed (hereinafter, such a substrate will be referred to as an "element substrate") in the figure, while a substrate 3 on which counter electrodes are formed (hereinafter, such a substrate will be referred to as a "counter substrate"), wherein the substrate 3 overlaps the substrate 2. This liquid crystal display device 1 is a reflective liquid crystal display device having a so-called H-VGA pixel arrangement of 480 dots (H).times.320 dots (V) for displaying a monochromatic image.
FIG. 24 is a plan view showing a pixel formed at an arbitrary position A (see FIG. 23) on the element substrate 2 within a display region of the liquid crystal display device 1. FIG. 25 is a plan view of the counter substrate 3 to be opposed to the element substrate 2. FIG. 26 is a cross-sectional view of the liquid crystal display device 1 taken along the line 26--26 shown in FIG. 24.
As shown in FIG. 24, one pixel includes: a pixel electrode 7 formed on the surface 2a of the element substrate 2 made of glass or the like; a signal line 5; and an MIM element 4.
Each of the signal lines 5 is connected to an associated element terminal 15 formed at the end of the element substrate 2. On the other hand, each of the counter electrodes 9 formed in a stripe shape on the surface 3a of the counter substrate 3 so as to cross the signal lines 5 at a right angle, is connected to an associated counter terminal 16 formed at the end of the counter substrate 3. The liquid crystal display device 1 is driven by applying a signal waveform to the element terminals 15 and the counter terminals 16.
Moreover, the MIM element 4 shown by cross-hatching in FIG. 24 should be designed so as to have a capacitance represented by an appropriate capacitance ratio with respect to the capacitance of the liquid crystal layer. A standard MIM element 4 is generally designed such that the ratio of the capacitance of the liquid crystal layer to that of the element 4 becomes approximately 10:1.
The lower electrode of the MIM element 4 is made of a material such as tantalum (Ta); the upper electrode thereof is made of titanium (Ti), aluminum (Al), chromium (Cr) or the like; and t,e insulating film thereof is made of tantalum oxide (TaO.sub.x), for example.
An exemplary method for fabricating the element substrate 2 will be described with reference to FIGS. 27A to 27D showing the cross section of the MIM element 4.
First, a Ta thin film is deposited on the surface 2a of the element substrate 2 made of glass or the like, by a sputtering method and then patterned by a photolithography method, thereby forming a signal line 5 and a lower electrode 5a (FIG. 27A).
Next, an insulating film 8 made of TaO.sub.x is formed on the Ta thin film by an anodization method or the like (FIG. 27B).
Subsequently, a thin film made of Ti or the like, is formed over the substrate 2, thereby forming a pattern for an upper electrode 6 (FIG. 27C).
Finally, a thin film made of a transparent conductive material such as indium-tin-oxide (ITO) is further formed thereon and then patterned by a photolithography method, thereby forming a pixel electrode 7 (FIG. 27D).
In such a configuration, the insulating film 8 exhibits nonlinear resistance properties and the MIM element 4 is formed at a site where the lower electrode 5a, the insulating film 8 and the upper electrode 6 are stacked.
As shown in FIG. 26, in order to control the orientations of liquid crystal molecules 14, an alignment film 10 (e.g., a polyimide film) is formed on the glass substrate (element substrate) 2 on which the MIM elements 4 are formed for the respective pixels, and the alignment film 10 is subjected to a rubbing treatment.
On the other hand, an alignment film 11 is also formed over the counter substrate 3 with the counter electrodes 9 formed on the surface 3a thereof. The alignment film 11 is subjected to a rubbing treatment in a direction perpendicular to the rubbing direction of the alignment film 10 of the element substrate 2.
The element substrate 2 and the counter substrate 3 are attached to each other via a seal member 17 (see FIG. 23) such that the alignment films 10 and 11 face each other and that a gap of about 10 .mu.m is provided therebetween. Next, liquid crystal material is injected into the gap and then sealed, thereby forming a liquid crystal cell.
Finally, as shown in FIG. 26, polarizers 12 and 13 are disposed on the outer surfaces of the liquid crystal cell (i.e., on the outer surface of the element substrate 2 and that of the counter substrate 3, respectively) such that the polarization axis of the polarizer 12 is perpendicular to that of the polarizer 13, thereby completing the liquid crystal display device 1. Herein, a polarizer with a reflector is used as the polarizer 12.
However, if a transmission type polarizer is attached as the polarizer 12 onto the outer surface of the element substrate 2 and a back light is provided, then a transmission type liquid crystal display device can be obtained. Furthermore, if a micro color filter layer is formed on the counter substrate 3, then color display can be performed.
Herein, the reflective liquid crystal display device 1 performs display by reflecting the externally incident light. Since a back light is unnecessary, the power consumption, the size and the weight of such a liquid crystal display device can be reduced. Thus, a reflective liquid crystal display device is expected to be widely used as a display device for portable information terminal units. A high-resolution large-capacity reflective liquid crystal display device allowing for the display of a so-called "paper white" type bright image is particularly suitable as a display device for portable information terminal units.
However, in such a reflective liquid crystal display device 1, since the incident light is absorbed by the polarizer 13, the reflectance thereof is generally decreased to about 50% or less. Thus, the brightness realized by such a liquid crystal display device is unsatisfactory.
In order to solve such a problem, liquid crystal display devices operating in a display mode in which the entire incident light can be efficaciously utilized without using polarizers have been proposed. Such liquid crystal display devices include, for example, a phase-change type liquid crystal display device operating in Guest Host (GH) mode.
FIG. 28 is a plan view showing the element substrate 2 of a phase-change type liquid crystal display device operating in GH mode. FIG. 29 is a plan view of the counter substrate 3 thereof. FIG. 30 is a cross-sectional view of the liquid crystal display device taken along the line 30--30 shown in FIG. 28.
It is noted that the basic planar configuration of the liquid crystal display device is similar to that shown in FIG. 23. Since this liquid crystal display device also has a similar H-VGA arrangement, the plan view thereof will not be depicted. However, in this liquid crystal display device, micro color filters 18a for cyan and micro color filters 18b for red are alternately provided in a checkerboard pattern on the counter substrate 3 for performing color display. Thus, the number of the element terminals necessary for the liquid crystal display device of this type becomes twice as large as that of the reflective liquid crystal display device: that is to say, 480.times.2=960.
In this liquid crystal display device, the pixel electrodes 7 also function as a reflector. In order to improve the luminance and the contrast ratio of the liquid crystal display device, the pixel electrodes 7 are formed on the upper surface of an organic insulating layer 19 on which uneven portions (i.e., the circles of various sizes drawn by the solid lines in FIG. 28) are formed. As a result, the upper surface of the pixel electrodes 7 becomes a diffusive reflective surface having a high reflectance. In other words, the reflector formed on the outer surface of the liquid crystal cell in the conventional reflective liquid crystal display device shown in FIG. 23 is formed inside the liquid crystal cell in the GH mode liquid crystal display device. The pixel electrodes functioning as a reflector is made of aluminum (Al) or the like.
Moreover, the upper electrode 6 of the MIM element 4 is electrically connected to a corresponding pixel electrode 7 via a through hole 20 provided through the organic insulating layer 19.
The ON/OFF states of the display are switched by controlling the orientation states of the Guest Host liquid crystal molecules including dichroic dyes upon an application of a voltage to the gap between the pixel electrodes 7 and the counter electrodes 9. As shown in FIG. 30, the molecules 21 of the dichroic dyes tend to be aligned with the liquid crystal molecules 22. Thus, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules 22 and the dichroic dye molecules 21 are arranged in a direction substantially vertical to the inward surfaces of the substrates so that the light incident from above the counter substrate 3 is passed through the liquid crystal layer without being absorbed into the dichroic dyes, reflected by the pixel electrodes 7 functioning as a reflector, passed through the liquid crystal layer again and then is emitted outward. On the other hand, when no voltage is applied to the liquid crystal layer, the liquid crystal molecules 22 and the dichroic dye molecules 21 are arranged irregularly so that the incident light is absorbed and cut off by the dichroic dye molecules 21. Since bright display (white display) or dark display (black display) is realized by selectively reflecting or cutting off the incident light, respectively, a bright image can be displayed without using any polarizers.
The basic equivalent circuit of such a phase-change type liquid crystal display device is similar to that of the reflective liquid crystal display device shown in FIG. 23. Thus, the phase-change type liquid crystal display device is also designed such that the ratio of the capacitance of the liquid crystal layer to that of the MIM element becomes an appropriate value (about 10:1).
FIG. 31A is an equivalent circuit diagram corresponding to one pixel of the liquid crystal display device using such MIM elements. An MIM element is represented as a parallel circuit formed by a nonlinear resistance R.sub.MIM and a capacitance C.sub.MIM, while the liquid crystal layer is represented as a parallel circuit formed by a resistance R.sub.LC and a capacitance C.sub.LC.
FIGS. 31B through 31E illustrate the basic waveforms of driving signals and the variations in voltages applied to the liquid crystal layer along the passage of time.
Assuming that a selection waveform having an amplitude V.sub.p is applied to the scanning lines (the counter electrodes) for a time period T.sub.ON every time a cycle T has passed as shown in FIG. 31B and that a data signal determining the display states of the liquid crystal layer is applied to the signal lines (lower electrodes) as shown in FIG. 31C. Then, the waveform of the actually applied voltage is obtained by combining the waveforms shown in FIG. 31B and 31C. Consequently, the waveform of the actually applied voltage becomes the waveform shown in FIG. 31D.
In general, in order to maintain the reliability of the liquid crystal molecules, an alternate current drive is performed by alternately inverting the polarities of the voltages to be applied.
Assuming that a selection voltage is applied to a scanning line (counter electrode), a voltage (V.sub.P .+-.V.sub.D) applied to the selected pixel is capacitance divided and a voltage V.sub.MIM applied to an MIM element is given by: EQU V.sub.MIM =C.sub.LC /(C.sub.LC +C.sub.MIM).multidot.(V.sub.P .+-.V.sub.D)
If the capacitance C.sub.MIM of the MIM element is set to be sufficiently small (i.e., C.sub.MIM &lt;&lt;C.sub.LC), almost all the voltage is applied to the MIM element. Since the MIM element has nonlinear current-voltage characteristics (i.e, the resistance of the element becomes low when an applied voltage is high), the MIMI element is turned ON so that charge corresponding to the display state is written onto the capacitance C.sub.LC of the liquid crystal layer.
When such a selection time T.sub.ON terminates at the trailing edge of the selection waveform, C.sub.LC and C.sub.MIM are capacitance coupled so that the voltage V.sub.LC applied to the liquid crystal layer is abruptly decreased by a difference .DELTA.V to be described later (see FIG. 31E). As a result, the voltage V.sub.LC is continuously discharged via the OFF resistance of the MIM element until the next selection time starts. EQU .DELTA.V=C.sub.MIM /(C.sub.LC +C.sub.MIM).multidot.(V.sub.P .+-.V.sub.D)
The display is performed while repeatedly applying similar signal waveforms from then on. During this display period, the ratio C.sub.LC /C.sub.MIM of the capacitance CLC of the liquid crystal layer to the capacitance C.sub.MIM of the MIM element is desirably as large as possible. This is because if the value of the ratio is small, then a sufficient amount of voltage cannot be applied to the MIM element during the selection time so that the element can not be perfectly turned ON. Furthermore, since the abrupt decrease .DELTA.V of the voltage applied to the liquid crystal layer becomes larger at the trailing edge of the selection waveform, several problems including a decrease in effective voltage applied to the liquid crystal layer are additionally caused. Thus, in order to display a satisfactory image, it is desirable to secure a standard capacitance ratio C.sub.LC /C.sub.MIM of about 10.
Furthermore, if the capacitance ratio C.sub.LC /C.sub.MIM becomes different among the respective pixels, then the displayed image becomes adversely non-uniform. Thus, it is necessary to eliminate a variation in capacitances of the MIM elements. If a thin insulating film having non-linearity is formed by an anodization method, then an insulating film having a satisfactorily uniform finish thickness can be obtained. Thus, in such a case, the variation in capacitances of the MIM elements substantially depends upon the finish area of each element. Consequently, in order to prevent an image from being non-uniformly displayed, it is important to fabricate elements of a uniform size.
FIG. 32 is a graph representing a relationship between a ratio of the capacitance C.sub.LC of the liquid crystal layer to the capacitance C.sub.MIM of the MIM element (hereinafter, simply referred to as a capacitance ratio C.sub.LC /C.sub.MIM) and the contrast ratio of the liquid crystal display device. Exemplary results obtained by measuring the characteristics of a phase-change type reflective liquid crystal display device operating in GH mode are represented in FIG. 32. Liquid crystal display devices having various capacitance ratios of about 5.98, about 7.82, about 10.84, about 12.88 and about 21.68 were modeled by intentionally varying the size (or the area) of an element while setting the electrode area of a pixel (i.e., the capacitance of the liquid crystal layer) to be constant.
The contrast ratio is saturated when the capacitance ratio C.sub.LC /C.sub.MIM is approximately 10 and tends to decrease as the capacitance ratio C.sub.LC /C.sub.MIM becomes smaller. The decrease in contrast ratio becomes remarkable particularly when the capacitance ratio C.sub.LC /C.sub.MIM is smaller than about 8. Thus, it is desirable to secure a capacitance ratio C.sub.LC /C.sub.MIM of about 8 or more, more preferably about 10.
Moreover, as the capacitance ratio C.sub.LC /C.sub.MIM becomes smaller, an element cannot be sufficiently turned ON unless the driving voltage is set to be high. Therefore, in view of the breakdown voltage of a driving LSI and the power consumption, a low driving voltage is desirable. Consequently, the capacitance ratio C.sub.LC /C.sub.MIM is preferably large.
In this case, since the thickness of the insulating film 8 of the MIM element 4 is about 40 nm to about 70 nm, the insulating film 8 has a low electric breakdown voltage and an insulation breakdown is easily caused by the static electricity generated during the fabrication process. As a result, the upper and the lower electrodes of the MIM element 4 are short-circuited and the MIM element can no longer normally function as a switching element. Since a pixel including such a short-circuited MIM element appears as a point defect on the screen, the display definition of the liquid crystal display device is degraded and the fabrication yield of the liquid crystal display device is adversely decreased.
A high-resolution large-capacity liquid crystal display device is required for portable information units in particular. Thus, the number of pixels or elements required for such a liquid crystal display device is necessarily increased. Furthermore, in the case of performing color display, even if the same number of pixels as that of the pixels used for monochromatic display are used, the number of the elements is further increased because a micro color filter layer is additionally provided.
Moreover, if a micro color filter layer is provided, then the point defects are displayed as colored points. As a result, since the point defects become more recognizable, the display definition is disadvantageously decreased. Therefore, it is desirable to suppress such point defects.
Various measures are taken for preventing the generation of static electricity. For example, the humidity during the fabrication process is monitored, and/or a ground or an ionizer is provided for the operator, thereby controlling the environment during the fabrication of a liquid crystal display device. Hc ever, it is currently impossible to completely prevent the insulation breakdown of an MIM element.
In consideration of these circumstances, measures for improving the breakdown voltage itself of an MIM element have also been taken. If the thickness of an insulating film 8 is increased, then it is surely possible to improve the electric breakdown voltage thereof. However, in such a case, the characteristics of such a switching element are deteriorated and the current-voltage characteristics thereof become less abrupt. Thus, the structure of an element has been reexamined instead of increasing the thickness of an insulator.
In an MIM element in general, an insulation breakdown is likely to be generated, because an electric field is likely to be concentrated on an insulator located over the etching edges of a lower electrode pattern or because another insulator has a poor step coverage when the insulator is formed. In an MIM element having a conventional structure, not only the insulator located on a flat portion of a lower electrode but also the insulator located over the etching edges are naturally used as insulators for the MIM element. Thus, the insulation breakdown of the MIM element itself is easily generated. Such an insulation breakdown generated along the periphery of the lower electrode makes defective an image displayed on a liquid crystal display device.
Thus, it has been proposed to further cover the insulator over the etching edges of a lower electrode, in which an insulation breakdown is easily generated, with an intermediate layer having insulation properties. In accordance with such a method, the insulation breakdown of an element is reduced by using only the insulator over the flat portion of the lower electrode as the insulator for an MIM element. MIM elements of this type are described, for example, in Japanese Laid-Open Patent Publications Nos. 1-270027, 3-160420, 4-367827 and 5-119353.
In general, an MIM element having such an intermediate insulating layer is fabricated by performing the steps of: forming a lower electrode; forming an insulator having non-linearity (or a first insulator); forming an intermediate insulating layer (or a second insulator); and forming an upper electrode. A so-called "top contact structure" in which the upper electrode and the lower electrode are connected to the insulator having non-linearity via an aperture-shaped opening (contact hole) provided in a part of the intermediate insulating layer is employed.
FIG. 33A is a plan view showing an MIM element in which a contact hole is provided through such an intermediate insulating layer. FIGS. 33B through 33D are cross-sectional views of the element substrates having various structures taken along the line 33B-D--33B-D shown in FIG. 33A. The cross section may have any of various kinds of layered structures depending upon the methods for fabricating the element. In any case, the cross section is characterized in that the etching edges of the lower electrode 5a are not used as a part of the element.
For example, the element shown in FIG. 33B can be fabricated by performing the following process steps.
First, a lower electrode 5a is formed on the surface of the element substrate 2 and then the surface of the lower electrode 5a is anodized, thereby depositing an insulator 8 having non-linearity thereon.
Next, an intermediate insulating layer 24 is deposited over the entire surface of the substrate 2 and an opening 23 to be a contact hole is provided through the insulating layer 24. In general, the intermediate insulating layer 24 is often made of a metal oxide or a metal nitride. It should be noted that the intermediate insulating layer 24 is required to be deposited at a low temperature. This is because if the previously deposited insulator 8 is exposed to a high temperature during a thermal process for depositing the intermediate insulating layer 24, the characteristics of the resulting MIM element are adversely deteriorated.
After the contact hole 23 has been provided, the upper electrode 6 is formed on the insulating layer 24, thereby forming an MIM element. Next, a pixel electrode 7 is formed thereon and is connected to the upper electrode 6. As a result, an MIM element is formed of the lower electrode 5a, the insulator 8 and the upper electrode 6. The area of the element becomes equal to the area of the contact hole 23.
In this case, in order to prevent the characteristics of the MIM element from being deteriorated owing to the heat applied when the intermediate insulating layer 24 is deposited, the order in which the insulator 8 and the intermediate insulating layer 24 are formed can be inverted.
For example, the element shown in FIG. 33C can be fabricated by performing the following process steps.
First, a lower electrode 5a is formed on the surface of the element substrate 2, an intermediate insulating layer 24 is deposited over the surface of the substrate 2 and an opening 23 to be a contact hole is provided through the insulating layer 24. Next, an insulator 8 having non-linearity is deposited on the surface of the insulating layer 24 by a sputtering method or the like and then patterned into a predetermined shape. Furthermore, an upper electrode 6 and a pixel electrode 7 are formed in the same way as in the case of forming the insulator 8 before the intermediate insulating layer 24 is formed.
In the thus-obtained MIM element shown in FIG. 33C, the area of the element is also equal to the area of the contact hole 23.
However, in the case of depositing the insulator 8 by a sputtering method, pin holes are likely to be generated. In addition, since the thickness of the insulator 8 becomes less uniform, a variation is adversely caused in capacities of the elements. Thus, it is possible to make the thickness of the insulator 8 more uniform if the insulator 8 having non-linearity is deposited by methods other than the method used in the process for fabricating the MIM element shown in FIG. 33C. For example, an anodization method, a thermal oxidization method or the like may be used instead of the sputtering method. In such a case, since the insulator 8 is deposited only over a part of the surface of the lower electrode 5a corresponding to the contact hole 23, the cross section of the MIM element has a structure shown in FIG. 33D.
As described above, an MIM element using a contact hole does not use the etching edges of the lower electrode, in which an insulation breakdown is likely to be caused, as a part of the element so that it is possible to suppress the possibility of the insulation breakdown. Thus, such a structure is effectively applicable to dealing with the defects.